Gauging the Sun: Comparative photometric and magnetic activity measurements of sunlike stars,

Transcription

1 Gauging the Sun: Comparative photometric and magnetic activity measurements of sunlike stars, G. W. Lockwood, J. C. Hall, & B. A. Skiff (Lowell Obs.) G. W. Henry (Tennessee State University) R. R. Radick (AFRL/VS) S. L. Baliunas, W. Soon, & R. A. Donahue (CfA)

2 Abstract Visible light photometric observations of a small sample of sunlike stars with mean chromospheric activity levels similar to or slightly lower than the Sun's suggest that total solar irradiance variations on activity cycle timescales may be comparatively small (Lockwood et al. 1992, Nature 360, 653; Radick et al. 1998, ApJS 118, 239). The Sun's irradiance variation over the past two cycles is 0.04% rms compared with 0.1% rms for the stellar sample measured at Lowell from 1984 to This assertion can now be tested using new photometric measurements from Fairborn Observatory automated telescopes ( ) that extend the duration of stellar observations to 17 years. Chromospheric activity measurements for these stars come from the Mount Wilson HK program ( ) and the Lowell Observatory Solar Stellar Spectrograph program ( ). In this presentation we will describe efforts to merge the overlapping Lowell and Fairborn photometry and the Mt. Wilson and Lowell HK measurements with the goal of reducing the uncertainties in previous efforts to characterize stellar photometric variations near the limit of detection.

3 The stellar sample Color-luminosity diagram for stars observed photometrically at Lowell and and Fairborn. Parallel Ca II H and K measurements for these stars are made at Mount Wilson and Lowell. Note the position of the Sun and the solar analog 18 Sco just below the center of the figure.

4 Filled circles indicate stars found to be photometrically variable from Lowell Observatory observations, Which stars vary? APT observations of the solar analog 18 Sco show no significant variations over 5 years. The ordinate scale, log R HK estimates the fraction of stellar flux emitted from the chromosphere. It is derived from the observed HK emission flux index, S.

5 The sense of variation Short Time Scale Cyclic Time Scale! Activity correlated with brightness variation (young stars) " Activity anti-correlated with brightness variation (Sun and solar age stars)

6 The Sun among the stars Magnetic activity on cycle timescales Cyclic magnetic activity, measured in the H and K lines of Ca II, obeys a power relation with respect to mean chromospheric activity. The Sun and the solar analog 18 Sco are slightly more active than average than stars of similar mean chromospheric activity.

7 The Sun among the stars Photometric activity on cycle timescales Cycle timescale photometric variations also follow a power law. Filled symbols indicate variability. The Sun s irradiance variations may be relatively small compared with stars of similar mean chromospheric activity. The solar analog 18 Sco may be even less variable than the Sun, although another decade of observation is needed to be sure. Drop lines indicate a correction for the variability of comparison stars.

8 The Sun: a cycling old star Total irradiance variation scaled to (b+y)/2 stellar mag scale Vital statistics B-V: 0.65 M v : 4.83 T eff : 5787 Sp: G2V <S>: 0.18 log R' HK : rms phot : rms CaII : Chromospheric variation -blue: NSO Ca II measurements -red: Lowell SSS K flux (scaled)

9 HD 1835: an active young star b,y brightness var. (mag) -green : Lowell -red : Fairborn APT Vital statistics B-V: 0.66 M v : 4.84 T eff : 5754 Sp: G2.5V <S>: 0.33 log R' HK : rms phot : rms CaII : Chromospheric variation -blue: Mt. Wilson HK prog. -red: Lowell SSS prog.

10 HD 10476: a cycling old star b,y brightness var. (mag) -green : Lowell -red : Fairborn APT Vital statistics B-V: 0.84 M v : 5.87 T eff : 5194 Sp: K1V <S>: 0.19 log R' HK : rms phot : rms CaII : Chromospheric variation -blue: Mt. Wilson HK prog. -red: Lowell SSS prog,

11 HD : the perfect solar twin? b,y brightness var. (mag) -green : Lowell -red : Fairborn APT Vital statistics B-V: 0.65 M v : 4.76 T eff : 5817 Sp: G2V <S>: 0.17 log R' HK : rms phot : rms CaII : Chromospheric variation -blue: Mt. Wilson HK prog. -yellow: Lowell SSS prog.

12 Conclusions 1. Many Sunlike stars display activity cycles 2. The Sun s photometric activity appears to be relatively small relative to some similar stars 3. The Sun is not unusual in terms of its magnetic activity behavior 4. Perfect solar twins appear to be relatively scarce in the solar neighborhood. 18 Sco is the currently favored best candidate 5. Data from two photometric programs can be successfully merged 6. Data from two spectroscopic programs can be successfully merged

13 Further reading A sample from the past 11 years Evidence of a pronounced activity cycle in the solar twin 18 Scorpii. Hall and Lockwood ApJ 545, L43-L45. Photometric and Ca II H and K spectroscopic variations in nearby Sun-like stars with planets. III. Henry, Baliunas,Donahue, Fekel, and Soon ApJ 415, Patterns of variation among Sunlike Stars. Radick, Lockwood, Skiff, Skiff, and Baliunas ApJS 118, The solar activity cycle. I. Observations of the end of cycle 22, 1993 September-1997 February. Hall and Lockwood ApJ 493, The photometric variability of Sun-like stars: Observations and results, Lockwood, Skiff, and Radick ApJ 485, Chromospheric variations in main-sequence stars. II. Baliunas, Donahue, Soon et al ApJ 438, Long-term solar brightness changes estimated from a survey of Sun-like stars. Lockwood, Skiff, Baliunas, and Radick Nature 360,

14 Data sources b, y photometry from the Lowell Observatory, b, y photometry from the Fairborn Observatory, Stellar Ca II S index from the Mount Wilson HK program, Stellar and solar HK fluxes from the Lowell Observatory Solar Stellar Spectrograph ( S 3 ) program, , converted to S Solar K emission equivalent width data from the National Solar Observatory, converted to S Funding credits Lockwood, Hall, and Skiff are supported by a grant from NSF. Baliunas, Donahue, and Soon are supported by the Air Force Office of Scientific Research Henry is supported by grants from NSF and NASA. Radick is supported by the Air Force Office of Scientific Research